1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting diode.
2. Related Art
A nitride semiconductor light emitting diode (hereafter abbreviated to ‘light emitting diode’) having a nitride semiconductor active layer is known in the art. As shown in FIG. 22(A), light projected from an active layer 132 of a light emitting diode 120 forms a spherical wave spreading in all directions. The intensity distribution of the projected light is a function of the cosine of the angle θ, as shown in FIG. 22(B). Directivity is not observed.
Since conventional light emitting diodes are mainly used for displays, there is no particular need to improve the directivity of the projected light from the light emitting diodes. Instead, the main aim of the light emitting diode is to improve visibility.
A light emitting diode is known wherein a reflecting mirror is formed on only one of the two sides of the active layer, this reflecting mirror being on the side opposite a light projecting face. In this light emitting diode, light emitted toward the side opposite the light projecting face is reflected toward the light projecting face by the reflecting mirror. This somewhat improves the directivity of the projected light, and ensures high visibility.
Further, a light emitting diode provided with a transparent resin mold having a lens attached thereto is known in the art. In this light emitting diode, the lens contained in the transparent resin mold somewhat improves the directivity of the projected light, and ensures high visibility.
Since the main aim in conventional light emitting diodes is to ensure high visibility, these conventional light emitting diodes have low directivity of the projected light, and the projected light spreads across a wide area. Although techniques of using a reflecting mirror or a lens to improve the directivity of the projected light are known, the directivity attained by this means remains unsatisfactory.
Light sources for optical communications require light with a high output and a high degree of directivity. For this, semiconductor lasers, which have a high output and a high degree of directivity, are used. However, there has been no particular need to increase directivity in nitride semiconductor light emitting diodes, since these are mainly used for displays, and research that satisfactorily increases directivity has not been performed.
We have recognized that short range optical communications need not utilize a high output semiconductor laser that requires a resonator structure, instead, a light emitting diode that does not require a resonator structure is often sufficient. The light emitting diode has a simpler structure than a semiconductor laser, the manufacturing process thereof is simpler, and the cost is lower. Consequently, there would be great merit in having the light source for short range optical communications comprising of, if possible, a light emitting diode, this being cheaper due to its not requiring the resonator structure, instead of the semiconductor laser, which is expensive due to its requiring the resonator structure.
In recent years, short range optical communications are being employed that utilize plastic optical fiber (hereafter, for convenience, referred to as ‘POF’). POF rather than silica glass is being utilized in short range optical communications because POF is (1) cheaper, (2) connecting operations are easier, etc. The POF utilized in these short range optical communications has high transmissivity in the range of visible light. Nitride semiconductor light emitting diodes, which emit short wavelengths (blue light and green light) of visible light, are suitable as a light source for POF.
Further, it is difficult to obtain good quality crystal growth with nitride semiconductors that emit short wavelengths (blue light and green light) of visible light, therefore, nitride semiconductor lasers are not yet practical.
From the viewpoint of wavelength of emitted light, the nitride semiconductor light emitting diode is suitable as a light source for POF, and additionally the price thereof is more advantageous than of the laser. However, since the directivity thereof is low, light fails to enter the optical fibers, with the result that coupling efficiency with optical fibers is low.
The light that is able to enter the optical fibers (represented here by POF) is restricted only to that light that is within the range of the angle of incidence of the optical fibers, this being determined by the configuration of the optical fibers (usually approximately ±15 degrees in the case of POF). When light projected by conventional light emitting diodes is to enter optical fibers, the light projected by the light emitting diode spreads widely beyond the range of the angle of incidence of the optical fibers, and the proportion of projected light unable to enter the optic fiber is high.
As shown in FIG. 23, the conventional light emitting diode has a rectangular first electrode 138 that supplies electric current flowing through an active layer. A second electrode 128 has an approximately semicircular shape, a portion of the rectangular first electrode 138 being cut away in a semicircular shape so as to surround the semicircular second electrode 128. Since almost all conventional light emitting diodes are used as displays, the light emitting area must be large. When a plurality of light emitting diodes formed on a substrate is to be separated into chips, cleavage or the like is normally used for separation, and consequently each chip is rectangular. A rectangular electrode is formed along each rectangular chip so as to increase the light emitting area thereof.
FIG. 24 shows numeric values for the intensity distribution of the emitted light of the conventional light emitting diode having the electrode configuration shown in FIG. 23. With this type of electrode configuration, an electric current does not flow with a uniform current density through the electrode 138, there being lower resistance in the region of the electrode 138 closer to the opposing electrode 128, and the current density consequently being higher. Light is emitted with higher intensity from the areas of the active layer that have a higher current density. Further, since electric fields are concentrated more readily in corners, the current density in the corners is higher, and the intensity of emitted light is higher. This is why, of the area of the electrode 138 closer to the opposing electrode 128, the corner areas thereof have a higher intensity of emitted light (see FIG. 24).
Transient behavior is also important, particularly in cases where modulation in light intensity is utilized, such as optical communications. However, if electric current first flows to areas in which the electric current flows most readily, the resistance in those areas drops, and a phenomenon is likely to occur whereby the electric current is concentrated in those areas.
FIG. 25 shows light projected from a conventional light emitting diode 120 entering an optical fiber. A high proportion of the light projected from the conventional light emitting diode 120 is unable to enter the optic fiber. One reason for this is that the directivity of the light projected from the conventional light emitting diode 120 is low, and so a large quantity of light leaks from the optical fiber. A second reason for this is that the intensity of light in the left half of FIG. 24 and the intensity of light in the right half thereof are asymmetrical. As shown in FIG. 25, the projected light distribution from the left half and right half areas of the active layer is polarized (polarized asymmetrically), a large quantity of the light emitted from the left half area of the active layer being projected outside the range of the angle of incidence of the optical fiber, as shown by FIG. 106. This is one reason why the proportion of light projected from the light emitting diode that is able to enter the optical fiber (i.e., the coupling efficiency of the light emitting diode with the optical fiber) is low.